The idea of a headwind when our desired course and the wind direction directly oppose each other is a relatively easy one to grasp: We don't need a wind triangle to understand that if the air in which we are flying is moving at 20 knots in a direction exactly opposite our heading, groundspeed will be reduced by 20 knots.
Instead, most pre-flight planning these days is conducted online, and well-refined algorithms compute the results of a wind triangle for us, providing projected groundspeed and magnetic heading to steer, all in a nice, concise flight log we can print at the FBO and carry with us to the airplane.
Regardless, if you're a little rusty on performing wind triangles on your E-6B, don't forget winds aloft are given in true values, and you'll eventually want to convert the results to a magnetic heading.